1. Field of the Invention
The present invention relates to a structural material for relieving or relaxing vibrations and impact of a structure or noises from the structure, and more specifically to a vibration-damping material and a fiber-reinforced composite material for absorbing vibrational energy generated during the operation of structures such as space structures, e.g., artificial satellites, office automation (OA) machinery and tools, various cars or equipments for leisure time amusement as well as a method for the preparation thereof. 2. Prior Art
Asphalt-based damping materials have been well-known and used for absorbing vibrational energy of buildings and cars. See, for instance, Japanese Unexamined Patent Publication Nos. Sho 57-29702, Sho 58-28034 and Sho 52-39723.
However, structures whose operation is accompanied by vibration have been diversified and the problem of vibration encountered in space structures such as artificial satellites and a variety of OA machinery and tools can not be used with conventional asphalt-based damping materials which give out a peculiar smell. Therefore, it has been desired to develop a damping material which can be preferably used even in structures operated within a closed spaces or in the vicinity of operators.
On the other hand, fiber-reinforced composite materials obtained by solidifying inorganic fibers such as carbon fibers and glass fibers and organic fibers such as aramid fibers as reinforcing fibers with a matrix resin such as an epoxy, polyimide or polyether ether ketone resin are lightweight and have high strength compared with the conventional metallic structural materials. In addition, desired mechanical properties can be imparted to these composite material by controlling the angle of orientation of these fibers. For this reason, these composite materials have been widely used for the production of various structural materials such as those for space structures, air planes and a variety of cars such as automobiles as well as equipment for leisure time amusement.
However, the fiber-reinforced composite materials are lightweight and low vibration-damping properties (for instance, a loss factor .eta. ranging from 0.001 to 0.01) approximately equal to that of the conventional metallic structural materials and accordingly the composite material easily cause vibrations. Therefore, these composite materials as such are insufficient and unacceptable for optimum use in the foregoing applications in which the quality is adversely affected by vibration, since the structures obtained from these composite materials vibrate with large amplitudes during operation thereof. Moreover, structures used in such applications are often produced through a one-piece molding method and accordingly the vibration damping (structural damping) due to friction at joined portions cannot be anticipated unlike the metallic structural materials. For this reason, the space structures such as artificial satellites suffer from various problems such as troubles of machinery and tools mounted thereon due to vibrations of the structures and a reduction of precision in positioning antennas. Consequently, these composite materials are not preferably used as structural materials for these applications. It has thus become a quite important problem in this field to impart vibration-damping properties to these fiber-reinforced composite materials.
To solve these problems, there has been investigated a method for improving the vibration-damping properties of composite materials through the improvement of vibration-damping properties of matrix resins used therein.
For instance, there has been proposed a method for improving vibration-damping properties which comprises sandwiching a film of a damping material, i.e., a resin exhibiting a high mechanical loss between two layers of a material obtained by impregnating reinforcing fibers with a matrix resin to give a multilayer structure.
As the foregoing vibration damping materials in the form of films, there have been used films mainly comprising thermoplastic resins such as polyolefins. However, it is difficult to obtain a large angle of energy loss (tan .delta.) through the use of a thermoplastic vibration damping film and it is likewise difficult to select the ingredient so that the temperature which provides a maximum value of the tan .delta. falling within the temperature range at which each structure is put into practical use. For this reason, there has been a need for the development of thermoplastic vibration damping films which can provide a large angle of energy loss (tan .delta.) and whose temperature capable of providing a maximum value of the tan .delta. is relatively easily be controlled.
It has, however, been confirmed that the foregoing method cannot provide any composite material having desired damping properties when both matrix and damping materials comprise thermoplastic resins. While not wanting to be bound by any particular theory, this is believed to be due to the following reasons.
The fiber-reinforced composite materials are in general used at a temperature ranging from -35.degree. C. to 35.degree. C. and accordingly a vibration damping material to be sandwiched is selected from those exhibiting high vibration-damping properties within the temperature range defined above.
The maximum angle of energy loss (tan .delta.) serving as an index of the vibration-damping properties of the composite materials is usually observed at the glass transition temperature (Tg) of a damping material used and thus it is preferred to use a damping material having a glass transition temperature falling within the range defined above.
On the other hand, the fiber-reinforced composite material per se must have excellent mechanical properties and as a result, preferably used are those having high strength and elastic modulus. Accordingly, materials having high glass transition points are necessarily used as matrix materials.
Upon preparing a fiber-reinforced composite material having excellent vibration-damping properties from matrix materials and damping materials which satisfy the requirements discussed above and which are both thermoplastic resins, these resins are intermingled during pressure molding with heating (during hardening). This leads to a shift of the glass transition point of the damping material towards the high temperature side and correspondingly desired vibration-damping properties cannot be ensured. Under these circumstances, an attempt has been made to eliminate these drawbacks by first completely hardening the damping material, then laminating the damping material with a matrix and pressure-molding the laminate under heating. However, this method likewise suffers from the foregoing problems and any damping material having desired properties is not obtained.
It would be assumed that the foregoing problems arise due to the diffusion of the matrix resin in the damping material although the glass transition point of the damping material is set to a level adjacent to room temperature and the damping material is completely hardened in advance.